Power amplifiers are known to produce a considerable amount of heat which causes a semiconductor device to heat up in the event the heat cannot be dissipated quickly enough. As semiconductor materials, such as silicon and germanium, have a negative temperature coefficient above a certain temperature, electric currents in the semiconductor device can progressively rise and irreversibly destroy the semiconductor device, a situation which is known as “thermal runaway”. It is therefore known to use in integrated circuits parallely arranged amplifier cells to enable distributing the currents and also to enable the dissipation of heat over a number of the amplifier cells. In an ideal world the currents and heat losses would be distributed equally. In practice, it is found that at least one of the parallely arranged amplifier cells may draw more current than others ones of the parallely arranged amplifier cells, due to small asymmetries in the parallely arranged amplifier cells. The asymmetries are self-energizing due to the negative temperature coefficient of semiconductors so that in the end one of the parallely arranged amplifier cells heats up more than the others, further increasing its collector current. Thus the largest fraction of a current might end up passing through only one of the several parallel amplifier cells, a situation commonly known as “current hogging”.
From EP 0 597 397 A2 a high power bipolar amplifier employed as a multi-finger structure with a plurality of transistor cells is known with integrated ballast resistances, such as resistors connected to each of a base finger. This kind of solution is especially effective in connection with HBTs, as the base fingers are generally electrically isolated allowing the possibility of an integrated base ballast resistor. As EP 0 597 397 points out, silicon bipolar transistors usually use integrated ballast resistances in the emitter finger as the base fingers are not electrically isolated.
In the event that one specific transistor cell is conducting more current than the other transistor cells this specific transistor cell will draw more base current than the other transistor cells. The increased base current will increase the voltage drop across the ballast resistance, thus causing a reduction of the base voltage. As a consequence the reduction of the base voltage reduces the current flowing through the collector of that specific transistor cell. Thus the ballast resistances counteract the current hogging, but too much of the ballasting leads to early compression.
The use of mobile communications networks has increased over the last decade. Operators of the mobile communications networks have increased the number of base transceiver stations in order to meet an increased demand for service by users of the mobile communications network. The operators of the mobile communications network wish to purchase components for the base transceiver stations at a lower price and also wish to reduce the running costs of the base transceiver station. Since power amplifiers consume more than 50% of the total power of a transmitter system, improvements in the efficiency and linearity of the power amplifier technology, such as preventing early compression in amplifier design are contributors to more efficient base transceiver stations and/or active antenna arrays.